Interest in wireless data communications has grown rapidly in the past few years due to the growth of the Internet. The nature of the data carried over a wireless network is highly determinative of the type of architecture required for efficient and reliable communications. The key to meeting the increasing demand for wireless services is the development of high performance radio systems that take the unique features of the data traffic into account. For example, the architecture of a real time communications system carrying voice and/or video diverges greatly from the design considerations for data communications systems. Compared to voice and other real-time traffic, data traffic usually has a minimum tolerance for transmission errors and a high tolerance of transmission delay. As a result, packet retransmissions are possible and often necessary. For data applications, the techniques of packet switching, dynamic resource assignment and link adaptation are more suitable than conventional techniques such as circuit switching, fixed resource allocation and fixed transmission schemes.
The need for re-engineering of communications systems in order to accommodate the needs of data traffic has been recognized. For example, several existing systems, such as Cellular Digital Packet Data (CDPD), Global System for Mobile Communication (GSM) and IS-136 have the capacity to support data services. However, these systems employ circuit switching (except for CDPD) and offer only low data rates. Currently, the data rate of GSM ranges from 2.4 kbps to 9.6 kbps. To enhance the data capability of GSM, a new service called the General Packet Radio Service (GPRS) has been proposed. In addition, the European Telecommunications Standards Institute (ETSI) has standardized a specification entitled Enhanced Data Rates for GSM Evolution (EDGE) as an attractive GSM evolution for providing broadband data services. Both EDGE and IS-136 utilize link adaptation in order to maximize throughput and promote bandwidth efficiency.
Link adaptation is a continuous process in which the attributes of each link within a communications system are dynamically updated to maximize throughput (or some other parameter) and efficiently utilize the available bandwidth according to a set of criteria. Typically, a link adaptation scheme consists of a set of modes each incorporating a different modulation/coding scheme or some other link parameter controlling the data rate. Each mode and corresponding modulation/coding scheme has an associated set of performance attributes. For example, the block error rate (BLER) is an important parameter in a link adaptation system. BLER is the probability that a block of bits transmitted from the receiver to the transmitter contains an error after decoding. BLER is a function of the signal-to-interference ratio (SIR) (ratio of signal to interference power) at the receiver such that each mode has a characteristic BLER curve as a function of SIR. Each modulation/coding scheme is also associated with a radio interface rate R, which is the actual rate of information transmission after accounting for the coding overhead. Using the performance attributes BLER and R for each mode, the throughput (measure of the actual bit transmission rate from transmitter to receiver) for each mode can be described as a function of SIR.
Link adaptation is accomplished by establishing a set of threshold values for choosing different transmission modes. These threshold values are used to determine the selection of each mode in the adaptation scheme based on some real time performance measure such as SIR. A link adaptation system operates by periodically taking a real time performance measure for each link (e.g., SIR at the receiver), comparing this performance measure with the threshold values for the modes and then selecting the appropriate mode that will maximize throughput.
The appropriate link adaptation threshold scheme is crucial to realize performance gains. If the thresholds are too aggressive, i.e., users with poor link quality select higher level modulation/coding schemes, the overall system performance will suffer due to excessive retransmissions. On the other hand, if these thresholds are too conservative, the system performance will also suffer due to low spectrum efficiency, which results in prolonged resource occupancy.
Traditional link adaptation schemes use BLER as a basis for establishing the set of adaptation thresholds. Usually the modulation/coding scheme is updated to maintain the BLER at a particular level, (e.g. 10%). Thus, the BLER establishes an acceptable level of error for a communications channel, which is appropriate for real time traffic. On the other hand, data services allow the retransmission of blocks in error at the cost of delay. Therefore, BLER is generally not the only criterion for data services since the ultimate measure for data services is throughput. The throughput depends upon BLER, the transmission rate and the possibility of retransmissions.
Link adaptation systems for data services typically rely upon throughput criteria to select the appropriate adaptation mode. For example, the central technology of EDGE is a link adaptation scheme that dynamically adapts the modulation/coding scheme according to the current link quality to maximize system throughput. EDGE incorporates two different modulation schemes, Offset Quadrature Phase Shift Keying (OQPSK) and Offset 16 Quadrature Amplitude Modulation (O16QAM). Combining these two different modulation schemes with four different coding schemes, EDGE supports a total of eight possible modulation/coding modes.
The set of thresholds comprising a link adaptation system is derived from a mathematical model of the wireless environment and the performance attributes for each modulation/coding mode. The choice of an appropriate wireless environment model is critical for establishing the correct link adaptation thresholds. For example, conventional link adaptation schemes such as EDGE are based on a model of a no-retransmission environment that assumes erroneous packets are discarded and do not increase the load in a system (i.e., packets are not retransmitted if lost or damaged in the transmission process).
However, retransmissions are in fact necessary for wireless data services and the behavior of a retransmission environment diverges significantly from a no-retransmission environment. In particular, a retransmission environment produces highly complex feedback behavior that can result in system instability and degraded performance. Failure to model this complex behavior and derive a correct set of link adaptation thresholds is a major shortcoming of traditional link adaptation schemes and can result in significantly degraded system performance and instability in the retransmission environment.
For example, retransmissions necessarily increase the load on the system, increase interference and lower the SIR. The lowering of the SIR will result in even more retransmissions until either the system reaches a steady state condition if it exists or the system becomes unstable. Thus, neglecting retransmissions significantly underestimates the interference in a wireless communications system and link adaptation schemes designed without considering retransmissions will perform poorly.